Counting device, physical quantity sensor, counting method and physical quantity measuring method

ABSTRACT

The counting device includes: a signal counter that counts the number of half cycles of input signals during given counting periods; a signal half cycle measurement unit that measures the half cycles; a frequency distribution generator that generates a frequency distribution of the half cycles; a representative value calculator configured to calculate a representative value of a distribution of the half cycles; a correction value calculator configured to calculate a total number Ns and a total number Nw n  so as to correct the number of the half cycles, wherein Ns represents the total of the number of the half cycles that are less than 0.5 times the represent value, and Nw n  represents the total of the number of the half cycles that are equal to or greater than 2n and less than (2n+2) times the representative value.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No.2009-181442, filed on Aug. 4, 2009, the entire contents of which arehereby incorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates to a counting device which counts thenumber of signals, and an interference type physical quantity sensorwhich measures the number of interference waveforms using the countingdevice and obtains physical quantities of an object to be measured.

2. Related Art

In the related art, there has been proposed a laser measurement deviceof a wavelength modulation type using a self-coupling effect of asemiconductor laser (see JP-A-2006-313080). FIG. 32 shows aconfiguration of such a laser measurement device. As shown in FIG. 32,the laser measurement device includes a semiconductor laser 201 whichemits laser light to an object 210, a photodiode 202 which convertslight output of the semiconductor laser 201 into an electrical signal, alens 203 which condenses light from the semiconductor laser 201, whichis then emitted to the object 210, and condenses return light from theobject 210, which is then incident into the semiconductor laser 201, alaser driver 204 which drives the semiconductor laser 201 toalternatively repeat a first oscillation period during which anoscillation wavelength continuously increases and a second oscillationperiod during which the oscillation wave length continuously decreases,a transimpedance amplifier 205 which converts an output current of thephotodiode 202 into a voltage and amplifies the voltage, a signalextraction circuit 206 which differentiates an output voltage of thetransimpedance amplifier 205 twice, a counting device 207 which countsthe number of mode hop pulses (MHPs) contained in an output voltage ofthe signal extraction circuit 206, a computing device 208 whichcalculates a distance to the object 210 and a speed of the object 210,and a display 209 which displays a result of the calculation by thecomputing device 208.

The laser driver 204 supplies a triangular wave driving current withrepeated increase/decrease at a constant rate of change in terms oftime, as an injection current, to the semiconductor laser 201. Thus, thesemiconductor laser 201 is driven to alternate between the firstoscillation period during which the oscillation wavelength continuouslyincreases at a constant rate of change and the second oscillation periodduring which the oscillation wavelength continuously decreases at aconstant rate of change. FIG. 33 shows a temporal change of theoscillation wavelength of the semiconductor laser 201. In FIG. 33, P1,P2, λa, λb, and Tt represent the first oscillation period, the secondoscillation period, the minimum value of the oscillation wavelength foreach oscillation period, the maximum value of the oscillation wavelengthfor each oscillation period, and a cycle of a triangular wave,respectively.

Laser light emitted from the semiconductor laser 201 is condensed by thelens 203 and then is incident into the object 210. Light reflected bythe object 210 is condensed by the lens 203 and then is incident intothe semiconductor laser 201. The photodiode 202 converts light output ofthe semiconductor laser 201 into a current. The transimpedance amplifier205 converts an output voltage of the photodiode 202 into a voltage andamplifies the voltage, and the signal extraction circuit 206differentiates an output signal of the transimpedance amplifier 205twice. The counting device 207 counts the number of MHPs, which arecontained in an output voltage of the signal extraction circuit 206, foreach of the first oscillation period P1 and the second oscillationperiod P2. The computing device 208 calculates a distance to the object210 and a speed of the object 210 based on the minimum oscillationwavelength λa and the maximum oscillation wavelength λb of thesemiconductor laser 201, the number of MHPs for the first oscillationperiod P1, and the number of MHPs for the second oscillation period P2.When the number of MHPs is measured using the technology of such aself-coupling type laser measurement device, it is possible to calculatea vibration frequency of the object from the number of MHPs.

The above-mentioned laser measurement device has problems in that errorsoccur in physical quantities such as the calculated distance, thecalculated vibration frequency due to an error of the number of Mil-IPscounted by the counting device, which occurs when noise, such asdisturbance light or the like, is counted as MHPs or uncountable MHPsare present due to omission of signals.

Therefore, the present inventor(s) has suggested a counting device whichis capable of eliminating an effect of deficient counting or excessivecounting by measuring a cycle of MHPs during a counting period,generating a frequency distribution of the cycle during the countingperiod from a result of the measurement, calculating a representativevalue of the cycle of MHPs from the frequency distribution, obtainingthe total sum Ns of frequencies of a class which is equal to or lessthan a first predetermined multiple of the representative value, and thetotal sum Nw of frequencies in a class which is equal to or more than asecond predetermined multiple of the representative value from thefrequency distribution, and correcting a counting result of MHPs basedon these frequencies Ns and Nw (see JP-A-2009-47676).

The counting device disclosed in JP-A-2009-47676 can achieve a generallygood correction as long as a SNR (Signal to Noise Ratio) is notextremely lowered.

However, with the counting device disclosed in JP-A-2009-47676, if asignal strength is extremely high in short range measurement as comparedto a hysteresis width, there may be a case where chattering occurs neara binarization threshold value in a signal input to the counting devicedue to noise having a frequency higher than that of MHP and signalshaving a short cycle or signals having a cycle which is about half theoriginal cycle of MHP are frequently generated. In this case, since thecycle shorter than the original cycle of MHP becomes a representativevalue of a distribution of cycle, there arises a problem in that acounting result of MHP can not be properly corrected and becomes largerby several times or so than its original value.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention address the abovedisadvantages and other disadvantages not described above. However, thepresent invention is not required to overcome the disadvantagesdescribed above, and thus, an exemplary embodiment of the presentinvention may not overcome any of the disadvantages described above.

Accordingly, it is an illustrative aspect of the present invention toprovide a counting device and a counting method which are capable ofcorrecting a counting error even when a high frequency noisecontinuously occurs in a signal input to the counting device, and aphysical quantity sensor and a physical quantity measuring method whichare capable of improving precision of measurement on physical quantitiesby correcting a counting error of MHP.

According to one or more illustrative aspects of the present invention,there is provided a counting device which counts the number of signalshaving a linear relationship with a specific physical quantity, whereinthe signals have substantially a single frequency if the specificphysical quantity is constant. The device includes: a signal counterconfigured to count the number of half cycles of input signals duringgiven counting periods; a signal half cycle measurement unit configuredto measure the half cycles of the input signals during the givencounting periods whenever a half cycle of the signal is input; afrequency distribution generator configured to generate a frequencydistribution of the half cycles of the input signals during the givencounting periods, based on a measurement result from the signal halfcycle measurement unit; a representative value calculator configured tocalculate a representative value of a distribution of the half cycles ofthe input signals, based on the frequency distribution; and a correctionvalue calculator configured to calculate, based on the measurementresult from the signal half cycle measurement unit, a total number Nsand a total number Nw_(n) so as to correct the number of the half cyclesof the input signals counted by the signal counter, wherein Nsrepresents the total of the number of the half cycles that are less than0.5 times the represent value, and Nw_(n) represents the total of thenumber of the half cycles that are equal to or greater than 2n and lessthan (2n+2) times the representative value, where n is a natural numberof 1 or more.

According to one or more illustrative aspects of the present invention,the correction value calculator is configured to calculate the correctednumber N′ of the half cycles of the input signal, based on the followingexpression:

$N^{\prime} = {\frac{1}{2}\left\{ {N - {Ns} + {\sum\limits_{n = 1}^{n_{\max}}\left( {2\; n \times {Nw}_{n}} \right)}} \right\}}$$n_{\max} \leqq \frac{T_{\max}}{T\; 0}$

wherein the number of the half cycles of the input signals counted bythe signal counter is N, the representative value is T0 and the maximumvalue of the half cycles of the input signals is T_(max).

According to one or more illustrative aspects of the present invention,the device further includes: a signal combination unit configured tocombine a first half cycle that is less than 0.5 times therepresentative value with at least one of second half cycles measuredbefore and after the first half cycle so as to set the combined halfcycle as a new half cycle of a signal waveform, wherein the signalcounter is configured to count the number of the half cycles containingthe new half cycles combined by the signal combination unit.

According to one or more illustrative aspects of the present invention,when the combined half cycle is less than 0.5 times the representativevalue, the signal combination unit continues to perform the combinationprocess until newly-combined half cycle is equal to or greater than 0.5times the representative value.

According to one or more illustrative aspects of the present invention,when the first half cycle that is less than 0.5 times the representativevalue is between an m-th half cycle Tm and a p-th half cycle Tp, whereTm and Tp are equal to or greater than 0.5 times the representativevalue, and m and p are natural numbers, the signal combination unit isconfigured to: combine half cycles ranging from the half cycle Tm to thehalf cycle Tp so as to set the combined cycle as the m-th half cycle Tm,if (m+p) is an even number; and combine half cycles ranging from thehalf cycle Tm to a half cycle Tn−1 so as to set the combined cycle asthe m-th half cycle Tm, if (m+p) is an odd number.

According to one or more illustrative aspects of the present invention,the representative value is one of the median, the mode, the mean and aclass value, the class value having a maximum value obtained bymultiplying the class value by a frequency corresponding to the classvalue.

According to one or more illustrative aspects of the present invention,there is provided a counting device which counts the number of signalshaving a linear relationship with a specific physical quantity, whereinthe signals have substantially a single frequency if the specificphysical quantity is constant. The device includes: a signal cyclemeasurement unit configured to measure cycles of input signals duringgiven counting periods whenever a signal is input; a frequencydistribution generator configured to generate a frequency distributionof the cycles of the input signals during the given counting periods,based on a measurement result from the signal cycle measurement unit; arepresentative value calculator configured to calculate a representativevalue of a distribution of the cycles of the input signals, based on thefrequency distribution; a signal combination unit configured to combinea first cycle that is less than 0.5 times the representative value witha cycle measured immediately after the first cycle so as to set thecombined cycle as a new cycle of a signal waveform, wherein when thecombined cycle is less than 0.5 times the representative value, thesignal combination unit continues to perform the combination processuntil newly-combined cycle is equal to or greater than 0.5 times therepresentative value; a signal counter configured to count the number ofthe cycles of the input signals during the given counting period,wherein the number of the cycles contains the new half cycles combinedby the signal combination unit; and a correction value calculatorconfigured to calculate, based on the measurement result from the signalcycle measurement unit, a total number Ns and a total number Nw_(n) soas to correct the number of the cycles of the input signals counted bythe signal counter, based on the total number Ns and the total numberNw_(n), wherein Ns represents the total of the number of the half cyclesthat are less than 0.5 times the represent value, and Nw_(n) representsthe total of the number of the half cycles that are equal to or greaterthan (n+0.5) and less than (n+1.5) times the representative value, wheren is a natural number of 1 or more.

According to one or more illustrative aspects of the present invention,the correction value calculator is configured to calculate the correctednumber N′ of the cycles of the input signals, based on the followingexpression:

$N^{\prime} = {N - {Ns} + {\sum\limits_{n = 1}^{n_{\max}}\left( {n \times {Nw}_{n}} \right)}}$$n_{\max} \leqq \frac{T_{\max}}{T\; 0}$

wherein the number of the cycles of the input signals counted by thesignal counter is N, the representative value is T0 and the maximumvalue of the cycles of the input signals is T_(max).

According to one or more illustrative aspects of the present invention,the representative value is one of the median, the mode, the mean and aclass value, the class value having a maximum value obtained bymultiplying the class value by a frequency corresponding to the classvalue.

According to one or more illustrative aspects of the present invention,there is provided a physical quantity sensor. The physical quantitysensor includes: a semiconductor laser which emits laser light to anobject to be measured; an oscillation wavelength modulator configured tooperate the semiconductor laser such that at least one of a firstoscillation period and a second oscillation period repeatedly exists,wherein an oscillation wavelength continuously and monotonicallyincreases during the first oscillation period, and the oscillationwavelength continuously and monotonically decreases during the secondoscillation period; a detector configured to detect electrical signalsincluding interference waveforms, the interference waveforms beingcaused by a self-coupling effect of the laser light emitted from thesemiconductor laser and return light from the object; the countingdevice, which counts the number of interference waveforms, wherein theinput signals are the electrical signals outputted from the detector,and the given counting periods are the first and second oscillationperiods; and a computing unit configured to calculate a physicalquantity of the object based on a counting result from the countingdevice.

According to one or more illustrative aspects of the present invention,there is provided a method of counting the number of signals having alinear relationship with a specific physical quantity, wherein thesignals have substantially a single frequency if the specific physicalquantity is constant. The method includes: (a) counting the number ofhalf cycles of an input signal during given counting periods; (b)measuring the half cycles of the input signals during the given countingperiods whenever a half cycle of the signal is input; (c) generating afrequency distribution of the half cycles of the input signals duringthe given counting periods, based on a measurement result in the step(b); (d) calculating a representative value of a distribution of thehalf cycles of the input signals, based on the frequency distribution;and (e) calculating, based on the measurement result in step (b), atotal number Ns and a total number Nw_(n) so as to correct the number ofthe half cycles of the input signals in step (a), wherein Ns representsthe total of the number of the half cycles that are less than 0.5 timesthe represent value, and Nw_(n) represents the total of the number ofthe half cycles that are equal to or greater than 2n and less than(2n+2) times the representative value, where n is a natural number of 1or more.

According to one or more illustrative aspects of the present invention,step (e) comprises: calculating the corrected number N′ of the halfcycles of the input signal, based on the following expression:

$N^{\prime} = {\frac{1}{2}\left\{ {N - {Ns} + {\sum\limits_{n = 1}^{n_{\max}}\left( {2\; n \times {Nw}_{n}} \right)}} \right\}}$$n_{\max} \leqq \frac{T_{\max}}{T\; 0}$

wherein the number of the half cycles of the input signals counted instep (a) is N, the representative value is T0 and the maximum value ofthe half cycles of the input signals is T_(max).

According to one or more illustrative aspects of the present invention,the method further includes: (f) combining a first half cycle that isless than 0.5 times the representative value with at least one of secondhalf cycles measured before and after the first half cycle so as to setthe combined half cycle as a new half cycle of a signal waveform,wherein step (a) comprises: counting the number of the half cyclescontaining the new half cycles combined by the signal combination unit.

According to one or more illustrative aspects of the present invention,when the combined half cycle is less than 0.5 times the representativevalue, step (f) comprises: continuing to perform the combination processuntil newly-combined half cycle is equal to or greater than 0.5 timesthe representative value.

According to one or more illustrative aspects of the present invention,when the first half cycle that is less than 0.5 times the representativevalue is between an m-th half cycle Tm and a p-th half cycle Tp, whereTm and Tp are equal to or greater than 0.5 times the representativevalue, and m and p are natural numbers, step (f) includes: combininghalf cycles ranging from the half cycle Tm to the half cycle Tp so as toset the combined cycle as the m-th half cycle Tm, if (m+p) is an evennumber; and combining half cycles ranging from the half cycle Tm to ahalf cycle Tn−1 so as to set the combined cycle as the m-th half cycleTm, if (m+p) is an odd number.

According to one or more illustrative aspects of the present invention,the representative value is one of the median, the mode, the mean and aclass value, the class value having a maximum value obtained bymultiplying the class value by a frequency corresponding to the classvalue.

According to one or more illustrative aspects of the present invention,there is provided a method of counting the number of signals having alinear relationship with a specific physical quantity, wherein thesignals have substantially a single frequency if the specific physicalquantity is constant. The method includes: (a) measuring cycles of inputsignals during given counting periods whenever a signal is input; (b)generating a frequency distribution of the cycles of the input signalsduring the given counting periods, based on a measurement result in step(a); (c) calculating a representative value of a distribution of thecycles of the input signals, based on the frequency distribution; (d)combining a first cycle that is less than 0.5 times the representativevalue with a cycle measured immediately after the first cycle so as toset the combined cycle as a new cycle of a signal waveform, wherein whenthe combined cycle is less than 0.5 times the representative value,wherein step (d) comprises: continuing to perform the combinationprocess until newly-combined cycle is equal to or greater than 0.5 timesthe representative value; (e) counting the number of the cycles of theinput signals during the given counting period, wherein the number ofthe cycles contains the new half cycles combined in step (d); and (f)calculating, based on the measurement result in step (a), a total numberNs and a total number Nw_(n) so as to correct the number of the cyclesof the input signals counted in step (e), wherein Ns represents thetotal of the number of the half cycles that are less than 0.5 times therepresent value, and Nw_(n) represents the total of the number of thehalf cycles that are equal to or greater than (n+0.5) and less than(n+1.5) times the representative value, where n is a natural number of 1or more.

According to one or more illustrative aspects of the present invention,step (f) includes: calculating the corrected number N′ of the halfcycles of the input signal, based on the following expression:

$N^{\prime} = {N - {Ns} + {\sum\limits_{n = 1}^{n_{\max}}\left( {n \times {Nw}_{n}} \right)}}$$n_{\max} \leqq \frac{T_{\max}}{T\; 0}$

wherein the number of the cycles of the input signals counted in step(e) is N, the representative value is T0 and the maximum value of thecycles of the input signals is T_(max).

According to one or more illustrative aspects of the present invention,the representative value is one of the median, the mode, the mean and aclass value, the class value having a maximum value obtained bymultiplying the class value by a frequency corresponding to the classvalue.

According to one or more illustrative aspects of the present invention,there is provided a physical quantity measuring method. The methodincludes: (a) operating a semiconductor laser, which emits laser lightto an object to be measured, such that at least one of a firstoscillation period and a second oscillation period repeatedly exists,wherein an oscillation wavelength continuously and monotonicallyincreases during the first oscillation period, and the oscillationwavelength continuously and monotonically decreases during the secondoscillation period; (b) detecting electrical signals includinginterference waveforms, the interference waveforms being caused by aself-coupling effect of the laser light emitted from the semiconductorlaser and return light from the object; (c) counting the number ofinterference waveforms in accordance with the above-mentioned method,wherein the input signals are the electrical signals, and the givencounting periods are the first and second oscillation periods; and (d)calculating a physical quantity of the object based on a counting resultobtained in step (c).

According to the present invention, by counting the number of halfcycles of the input signal during the counting period, measuring thehalf cycle of the input signal during the counting period, generatingthe frequency distribution of the half cycle of the input signal duringthe counting period from this measurement result, calculating therepresentative value of the distribution of half cycles of the inputsignal from the frequency distribution, obtaining the total sum Ns ofthe number of half cycles which are smaller than ½ of the representativevalue, and the total sum Nw_(n) of the number of half cycles, which areequal to or larger than 2n times and smaller than (2n+2) times therepresentative value, and correcting the counting result of the signalcounter based on the frequencies Ns and Nw_(n), a counting error can becorrected with high precision even when high frequency noisecontinuously occurs in a signal input to the counting device.

In addition, in the present invention, by providing the signalcombination unit for taking a cycle, which is a combination of a halfcycle which is smaller than ½ of the representative value and at leastone of half cycles measured before and after that, as a half cycle aftercombination for the measurement result of the signal half cyclemeasurement unit, and taking a signal waveform generated by combiningcycles as a waveform corresponding to a half cycle of one signal, and bycounting the number of signals after processing of the signalcombination unit during the counting period, instead of counting thenumber of input signals, by unit of the signal counter, a counting errorcan be further reduced.

In addition, in the present invention, for the measurement result of thesignal half cycle measurement unit, as the signal combination unitperforms a process of taking a cycle, which is a combination of a halfcycle smaller than ½ of the representative value and a half cyclemeasured immediately thereafter, as a half cycle after combination, andtaking a signal waveform generated by combining cycles as a waveformcorresponding to a half cycle of one signal, until the half cycle aftercombination reaches ½ or more of the representative value, a countingerror can be further reduced.

In addition, in the present invention, for the measurement result of thesignal half cycle measurement unit, if a half cycle smaller than ½ ofthe representative value lies between an m-th half cycle Tm which isequal to or larger than ½ of the representative value and a p-th halfcycle Tp which is equal to or larger than ½ of the representative value(m and p are natural numbers), as the signal combination unit takes acycle, which is a combination of from the half cycle Tm to the halfcycle Tp, as a half cycle after combination if (m+p) is an even number,takes a cycle, which is a combination of from the half cycle Tm to ahalf cycle Tn−1, as a half cycle after combination if (m+p) is an oddnumber, and takes a signal waveform generated by combining cycles as awaveform corresponding to the m-th half cycle, a counting error can bereduced even if burst noise or popcorn noise having a cycle which isequal to or larger than a ¼ cycle of a signal of an object to bemeasured is mixed in a signal input to the counting device.

In addition, in the present invention, by measuring a cycle of an inputsignal during a counting period, generating a frequency distribution ofthe cycle of the input signal during the counting period from thismeasurement result, calculating a representative value of a distributionof the cycle of the input signal from the frequency distribution,performing a process of taking a cycle, which is a combination of acycle which is smaller than ½ of the representative value and a cyclemeasured immediately thereafter, as a cycle after combination for thecycle measurement result, and taking a signal waveform generated bycombining cycles as a waveform corresponding to one cycle of one signal,until the cycle after combination reaches ½ or more of therepresentative value, counting the number of signals after processing ofthe signal combination unit during the counting period, obtaining thetotal sum Ns of the number of cycles which are smaller than ½ of therepresentative value, and the total sum Nw_(n) of the number of cycles,which are equal to or larger than (n+0.5) times and smaller than (n+1.5)times the representative value calculated by the second representativevalue calculating unit, from a processing result of the signalcombination unit, and correcting a counting result of the signal counterbased on the frequencies Ns and Nw_(n), a counting error can becorrected with high precision even when high frequency noisecontinuously occurs in a signal input to the counting device.

In addition, in the present invention, by using the counting devicewhich is capable of correcting a counting error with high precision, itis possible to measure physical quantities of an object to be measuredwith high precision even if noise having a frequency higher than that ofan interference waveform continuously occurs in a signal input to thecounting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a vibrationfrequency measurement device according to a first embodiment of thepresent invention;

FIGS. 2A and 2B are waveform diagrams schematically showing an outputvoltage waveform of a transimpedance amplifier and an output voltagewaveform of a filter according to the first embodiment of the presentinvention;

FIG. 3 is a view for explaining a mode hop pulse;

FIG. 4 is a view showing the relationship between an oscillationwavelength of a semiconductor laser and an output waveform of aphotodiode;

FIG. 5 is a flow chart showing operation of a counting device and acomputing device according to the first embodiment of the presentinvention;

FIG. 6 is a block diagram showing one example of a configuration of thecounting device according to the first embodiment of the presentinvention;

FIG. 7 is a flow chart showing operation of the counting deviceaccording to the first embodiment of the present invention;

FIG. 8 is a block diagram showing one example of a configuration of acounting result corrector of the counting device according to the firstembodiment of the present invention;

FIGS. 9A to 9D are views for explaining operation of a counter of thecounting device according to the first embodiment of the presentinvention;

FIG. 10 is a view for explaining operation of a half cycle measuringunit of the counting device according to the first embodiment of thepresent invention;

FIG. 11 is a view for explaining the relationship between a periodduring which a representative value calculator of the counting devicecalculates a representative value and a counting period of a correctionobject according to the first embodiment of the present invention;

FIG. 12 is a block diagram showing one example of a configuration of thecomputing device according to the first embodiment of the presentinvention;

FIGS. 13A to 13C are views for explaining operation of a binarizer ofthe computing device according to the first embodiment of the presentinvention;

FIG. 14 is a view for explaining operation of a cycle measuring unit ofthe computing device according to the first embodiment of the presentinvention;

FIG. 15 is a view showing one example of frequency distribution of acycle of a binarization output produced by binarizing a counting resultof the counting device according to the first embodiment of the presentinvention;

FIG. 16 is a schematic view showing the frequency used for correction ofa counting result of a counter of the computing device according to thefirst embodiment of the present invention;

FIGS. 17A and 17B are views for explaining the principles of correctingthe counting result of the counter of the computing device according tothe first embodiment of the present invention;

FIGS. 18A to 18D are views for explaining problems of a related-artcounting device;

FIG. 19 is a view showing one example of frequency distribution of acycle of a mode hop pulse when high frequency noise is mixed to a signalinput to the counting device;

FIG. 20 is a view showing one example of a frequency distribution of ahalf cycle of the mode hop pulse;

FIG. 21 is a view showing a frequency distribution of a cycle generatedaccording to the binarization output shown in FIG. 18;

FIGS. 22A to 22D are views for explaining signals obtained by thevibration frequency measurement device according to the first embodimentof the present invention when a ratio of the maximum rate of vibrationof an object to a distance to the object is larger than a rate of changein a wavelength of a semiconductor laser;

FIG. 23 is a block diagram showing one example of a configuration of acounting device according to a second embodiment of the presentinvention;

FIG. 24 is a block diagram showing one example of a configuration of acounting result corrector of the counting device according to the secondembodiment of the present invention;

FIG. 25 is a flow chart showing operation of the counting deviceaccording to the second embodiment of the present invention;

FIGS. 26A to 26C are views for explaining operation of a signal combinerof the counting device according to the second embodiment of the presentinvention;

FIGS. 27A to 27C are views for explaining operation of a signal combinerof a counting device according to a third embodiment of the presentinvention;

FIG. 28 is a block diagram showing one example of a configuration of acounting device according to a fourth embodiment of the presentinvention;

FIG. 29 is a block diagram showing one example of a configuration of acounting result corrector of the counting device according to the fourthembodiment of the present invention;

FIG. 30 is a flow chart showing operation of the counting deviceaccording to the fourth embodiment of the present invention;

FIGS. 31A to 31C are views for explaining operation of a signal combinerof the counting device according to the fourth embodiment of the presentinvention;

FIG. 32 is a block diagram showing a configuration of a related-artlaser measuring instrument; and

FIG. 33 is a view showing one example of temporal change of anoscillation wavelength of a semiconductor laser in the laser measuringinstrument shown in FIG. 32.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS First Embodiment

Hereinafter, embodiments of the present invention will be described withreference to the drawings. FIG. 1 is a block diagram showing aconfiguration of a vibration frequency measurement device according to afirst embodiment of the present invention.

Referring to FIG. 1, a vibration frequency measurement device includes asemiconductor laser 1 which emits laser light to an object 10 to bemeasured, a photodiode 2 which converts light output of thesemiconductor laser 1 into an electrical signal, a lens 3 whichcondenses light from the semiconductor laser 1, which is then emitted tothe object 10, and condenses return light from the object 10, which isthen incident into the semiconductor laser 1, a laser driver 4 whichserves as an oscillation wavelength modulator for driving thesemiconductor laser 1, a transimpedance amplifier 5 which converts anoutput current of the photodiode 2 into a voltage and amplifies thevoltage, a filter 6 which removes a carrier wave from an output voltageof the transimpedance amplifier 5, a counting device 7 which counts thenumber of mode hop pulses (MHPs), which are self-coupling signalscontained in the output voltage of the filter 6, a computing device 8which calculates a vibration frequency of the object 10 based on acounting result of the counting device 7, and a display 9 which displaysa measurement result of the computing device 8.

The photodiode 2 and the transimpedance amplifier 5 constitute adetector. Hereinafter, for the purpose of facilitation of description,it is assumed that the semiconductor laser 1 used is of a type having nomode hopping effect (for example, VCSEL type, DFB laser type, etc.).

The laser driver 4 supplies a triangular wave driving current withrepeated increase/decrease at a constant rate of change in terms oftime, as an injection current, to the semiconductor laser 1. Thus, thesemiconductor laser 1 is driven to alternate between a first oscillationperiod P1 during which the oscillation wavelength continuously increasesat a constant rate of change in proportion to the magnitude of theinjection current and a second oscillation period P2 during which theoscillation wavelength continuously decreases at a constant rate ofchange. In this case, the temporal change of the oscillation wavelengthof the semiconductor laser 1 is as shown in FIG. 33. In this embodiment,the maximum value λb of the oscillation wavelength, the minimum value λaof the oscillation wavelength and the difference λb−λa therebetween arealways constant.

Laser light emitted from the semiconductor laser 1 is condensed by thelens 3 and then is incident into the object 10. Light reflected by theobject 10 is condensed by the lens 3 and then is incident into thesemiconductor laser 1. However, light condensation by the lens 3 is notessential. The photodiode 2 is disposed within or near the semiconductorlaser 1 and converts a light output of the semiconductor laser 1 to acurrent. The transimpedance amplifier 5 converts an output current ofthe photodiode 2 into a voltage and amplifies the voltage.

The filter 6 is configured to extract a superpositional signal from amodulated wave. FIG. 2A is a waveform diagram schematically showing anoutput voltage waveform of the transimpedance amplifier 5 and FIG. 2B isa waveform diagram schematically showing an output voltage waveform ofthe filter 6. These figures show a procedure of extracting an MHPwaveform (interference waveform) shown in FIG. 2B by removing anoscillation waveform (carrier wave) of the semiconductor laser 1 from awaveform (modulated wave) of FIG. 2A corresponding to an output of thephotodiode 2.

Here MHP, which is a self-coupling signal, will be described. As shownin FIG. 3, assuming that the distance from a mirror layer 1013 to theobject 10 is L and the laser oscillation wavelength is λ, return lightfrom the object 10 and laser light within a photonic resonator of thesemiconductor laser 1 constructively interfere with each other when thefollowing resonance condition is satisfied, thereby slightly increasinglaser output.

L=qλ/2  (1)

In equation (1), q is an integer. This effect may be sufficientlyobserved through an amplification action which is caused as the apparentreflectivity in the resonator of the semiconductor laser 1 is increasedeven if scattered light from the object 10 is very weak.

FIG. 4 is a view showing the relationship between an oscillationwavelength of the semiconductor laser 1, which is changed at a constantrate, and an output waveform of the photodiode 2. If the equation (1),L=qλ/2, is satisfied, a phase difference between the return light andthe laser light in the photonic resonator becomes 0° (in phase), whichresults in the highest constructive interference between the returnlight and the laser light in the photonic resonator. However, ifL=qλ/2+λ/4, the phase difference reaches 180° (out of phase), whichresults in the lowest constructive interference between the return lightand the laser light in the photonic resonator. Accordingly, the laseroutput alternates between the strongest point and the weakest point uponvarying the oscillation wavelength of the semiconductor laser 1. At thistime, when the laser output is detected by the photodiode 2, a steppedwaveform with a constant cycle is obtained as shown in FIG. 4. Such awaveform is generally called an “interference pattern.” Every componentof the stepped waveform (or the interference pattern) is referred to as“MHP.” When the oscillation wavelength of the semiconductor laser 1 isvaried for a certain period of time, the number of MHPs is varied inproportion to a measurement distance.

Next, operation of the counting device 7 and the computing device 8 willbe described. FIG. 5 is a flow chart showing operation of the countingdevice 7 and the computing device 8.

The counting device 7 counts the number of MHPs included in the outputvoltage of the filter 6 for the first oscillation period P1 and thesecond oscillation period P2 (Step S1 in FIG. 5). FIG. 6 is a blockdiagram showing one example of a configuration of the counting device 7.The counting device 7 includes a binarizer 71, an AND gate 72, a counter73, a counting result corrector 74 and a storage 75. The transimpedanceamplifier 5, the filter 6, the binarizer 71, the AND gate 72 and thecounter 73, the latter 3 components of which are included in thecounting device 7, constitute a signal counter.

FIG. 7 is a flow chart showing the operation of the counting device 7and FIG. 8 is a block diagram showing one example of a configuration ofthe counting result corrector 74. The counting result corrector 74includes a half cycle measuring unit 740, a frequency distributiongenerator 741, a representative value calculator 742 and a correctionvalue calculator 743.

FIGS. 9A to 9D are views for explaining operation of the counter 73.FIG. 9A is a schematic view showing a waveform of the output voltage ofthe filter 6, that is, an MHP waveform, FIG. 9B is a view showing anoutput of the binarizer 71 corresponding to FIG. 9A, FIG. 9C is a viewshowing a gate signal GS input to the counting device 7 and FIG. 9D is aview showing a counting result of the counter 73 corresponding to FIG.9B.

First, the binarizer 71 of the counting device 7 determines whether ornot the output voltage of the filter 6 shown in FIG. 9A has a high level(H) or a low level (L) and outputs the result of the determination asshown in FIG. 9B. In this case, the binarizer 71 binarizes the outputvoltage of the filter 6 by determining that the output voltage of thefilter 6 has the high level (H) if it rises above a threshold value TH1and determining that the output voltage of the filter 6 has the lowlevel (L) if it falls below a threshold value TH2 (TH2<TH1).

The AND gate 72 outputs a result of AND operation of an output of thebinarizer 71 and the gate signal GS as shown in FIG. 9C and the counter73 counts the number of rises/falls of the output of the AND gate 72(FIG. 9D). Here, the gate signal GS is a signal which rises at the headof a counting period (the first oscillation period P1 or the secondoscillation period P2 in this embodiment) and falls at the end of thecounting period. Accordingly, the counter 73 counts the number ofrising/falling edges (i.e., the number of half cycles of MHPs) of theoutput of the AND gate 72 during the counting period (Step S100 in FIG.7).

FIG. 10 is a view for explaining operation of the half cycle measuringunit 740 of the counting result corrector 74. The half cycle measuringunit 740 measures half cycles of MHPs during the counting period (StepS101 in FIG. 7). Specifically, the half cycle measuring unit 740 detectsthe rise in the output of the AND gate 72 by comparing the output of theAND gate 72 during the counting period with a threshold value TH3, whiledetecting the fall in the output of the AND gate 72 by comparing theoutput of the AND gate 72 during the counting period with a thresholdvalue TH4. Then, the half cycle measuring unit 740 measures the halfcycle of the output of the AND gate 72 (that is, the MHP half cycle)during the counting period by measuring a period tud from the rising ofthe output of the AND gate 72 to the next falling and measuring a periodtdu from the falling of the output of the AND gate 72 to the nextrising. As such, the MHP half cycle refers to the period tud or tdu. Thehalf cycle measuring unit 740 performs the above-mentioned measurementwhenever one of the rising and falling of the output of the AND gate 72is detected.

The storage 75 stores the counting result of the counter 73 and themeasurement result of the half cycle measuring unit 740.

After the gate signal GS falls and the counting period expires, thefrequency distribution generator 741 of the counting result corrector 74generates a frequency distribution of the MHP half cycle during thecounting period from the measurement result of the half cycle measuringunit 740 stored in the storage 75 (Step S102 in FIG. 7).

Subsequently, the representative value calculator 742 of the countingresult corrector 74 calculates a representative value T0 of the MHP halfcycle from the frequency distribution generated by the frequencydistribution generator 741 (Step S103 in FIG. 7). Here, the mode, medianor mean of the MHP half cycle may be taken as the representative valueT0. Alternatively, the representative value calculator 742 may take aclass value, which gives the maximum of the product of the class valueand the frequency, as the representative value T0. Table 1 showsnumerical examples of the frequency distribution and products of classvalues and frequencies in these numerical examples.

TABLE 1 Class value 1 2 3 4 5 6 7 8 9 10 Frequency 11 2 0 3 7 10 6 2 3 1Product 11 4 0 12 35 60 42 16 27 10

Numerical Examples of the Frequency Distribution

In the example of Table 1, the mode (class value) having the maximumfrequency is 1. On the contrary, a class value giving the maximum of theproduct of the class value and the frequency is 6, which is differentfrom the mode. The reason why the class value giving the maximum of theproduct of the class value and the frequency is taken as therepresentative value T0 will be described later. The representativevalue T0 calculated by the representative value calculator 742 is storedin the storage 75. The representative value calculator 742 performs sucha calculation of the representative value T0 whenever the frequencydistribution is generated by the frequency distribution generator 741.

The correction value calculator 743 of the counting result corrector 74obtains the total sum Ns of the number of half cycles which are smallerthan ½ of the representative value T0, and the total sum Nw_(n) of thenumber of half cycles, which are equal to or larger than 2n times andsmaller than (2n+2) times (n is a natural number equal to or larger than1 and equal to or smaller than n_(max)) the representative value T0,from the measurement result of the half cycle measuring unit 740 andcorrects the counting result of the counter 73 as expressed by thefollowing equation (Step S104 in FIG. 7).

$\begin{matrix}{{N^{\prime} = {\frac{1}{2}\left\{ {N - {Ns} + {\sum\limits_{n = 1}^{n_{\max}}\left( {2\; n \times {Nw}_{n}} \right)}} \right\}}}{n_{\max} \leqq \frac{T_{\max}}{T\; 0}}} & (2)\end{matrix}$

In Equation (2), N is the number of MHP half cycles, which is thecounting result of the counter 73, N′ is the number of MHPs obtainedafter correction, and T_(max) is the maximum value which can be taken bythe MHP half cycle. The principle of correcting the counting result ofthe counter 73 will be described later.

The counting device 7 performs the above-described process for the firstoscillation period P1 and the second oscillation period P2.

In addition, the representative value T0 used by the correction valuecalculator 743 may use a value calculated from the measurement result ofthe half cycle measuring unit 740 in a counting period earlier by onecycle of a carrier wave (triangular wave) than a counting period of acorrection object or may use a value calculated from the measurementresult of the half cycle measuring unit 740 in the counting period forthe correction object. FIG. 11 is a view for explaining the relationshipbetween the period during which the representative value calculator 742calculates the representative value T0 and the counting period of thecorrection object, showing a temporal change of the oscillationwavelength of the semiconductor laser 1.

When the correction value calculator 743 uses the representative valueT0 calculated from the measurement result in the period earlier by onecycle of a carrier wave than the counting period of the correctionobject, it corrects the counting result of a first oscillation periodP1-2 using the representative value T0 calculated in a first oscillationperiod P1-1, for example, shown in FIG. 11 and corrects the countingresult of a second oscillation period P2-2 using the representativevalue T0 calculated in a second oscillation period P2-1. In addition,when the correction value calculator 743 uses the representative valueT0 calculated from the measurement result in the counting period of thecorrection object, it corrects the counting result of the firstoscillation period P1-1 using the representative value T0 calculated inthe first oscillation period P1-1, for example, shown in FIG. 11 andcorrects the counting result of the second oscillation period P2-1 usingthe representative value T0 calculated in the second oscillation periodP2-1.

However, even when the representative value T0 calculated from themeasurement result in the period of a carrier wave one cycle earlierthan the counting period of the correction object is used, since aninitial value of the representative value T0 in the first process doesnot exist, the counting result is corrected by obtaining therepresentative value T0 from the measurement result of the half cyclemeasuring unit 740 in the counting period of the correction object.

Next, the computing device 8 calculates the vibration frequency of theobject 10 based on the number of MHPs counted by the counting device 7.FIG. 12 is a block diagram showing one example of a configuration of thecomputing device 8. The computing device 8 includes a storage 80 whichstores the counting result of the counting device 7, and the like, abinarizer 81 which binarizes the counting result of the counting device7, a cycle measuring unit 82 which measures a cycle of a binary outputfrom the binarizer 81, a frequency distribution generator 83 whichgenerates a frequency distribution of the cycle of the binary output, areference cycle calculator 84 which calculates a reference cycle whichis a representative value of the frequency distribution of the cycle ofthe binary output, a counter 85 which is a binary output counting unitfor counting the number of pulses of the binary output, a corrector 86which corrects the counting result of the counter 85, and a frequencycalculator 87 which calculates the vibration frequency of the object 10based on the corrected counting result.

The counting result of the counting device 7 is stored in the storage 80of the computing device 8. The binarizer 81 of the computing device 8binarizes the counting result of the counting device 7, which was storedin the storage 80 (Step S2 in FIG. 5). FIGS. 13A to 13C are views forexplaining operation of the binarizer 81. FIG. 13A is a view showing atemporal change of the oscillation wavelength of the semiconductor laser1. FIG. 13B is a view showing the temporal change in the counting resultof the counting device 7. FIG. 13C being a view showing an output D(t)of the binarizer 81. In FIG. 13B, N′u is the counting result of thefirst oscillation period P1 and N′d is the counting result of the secondoscillation period P2.

The binarizer 81 compares the counting results, N′u and N′d, of the twooscillation periods P1 and P2, which are temporally adjacent to eachother, and binarizes these counting results. Specifically, the binarizer81 performs the following equations.

If N′u(t)≧N′d(t−1) then D(t)=1  (3)

If N′u(t)<N′d(t−1) then D(t)=0  (4)

If N′d(t)≦N′u(t−1) then D(t)=1  (5)

If N′d(t)>N′u(t−1) then D(t)=0  (6)

In Equations (3) to (6), (t) represents the number of MHPs measured atcurrent time t and (t−1) represents the number of MHPs measured oneperiod before the current time t. In Equations (3) and (4), the countingresult at the current time t is the counting result N′u of the firstoscillation period P1 and the counting result one period before thecurrent time t is the counting result N′d of the second oscillationperiod P2. In this case, the binarizer 81 takes the output D(t) at thecurrent time t as “1” (high level) if the counting result N′u(t) at thecurrent time t is equal to or larger than the counting result N′d(t−1)one period before the current time t, and takes the output D(t) at thecurrent time t as “0” (low level) if the counting result N′u(t) at thecurrent time t is smaller than the counting result N′d(t−1) one periodbefore the current time t.

In Equations (5) and (6), the counting result at the current time t isthe counting result N′d of the second oscillation period P2 and thecounting result one period before the current time t is the countingresult N′u of the first oscillation period P1. In this case, thebinarizer 81 takes the output D(t) at the current time t as “1” if thecounting result N′d(t) at the current time t is equal to or smaller thanthe counting result N′u(t−1) one period before the current time t, andtakes the output D(t) at the current time t as “0” if the countingresult N′d(t) at the current time t is larger than the counting resultN′u(t−1) one period before the current time t.

Thus, the counting results of the counting device 7 are binarized. Theoutput D(t) of the binarizer 81 is stored in the storage 80. Thebinarizer 81 performs the above-described binarizing process each timeat which the number of MHPs is measured by the counting device 7 (everyoscillation period).

The binarization of the counting result of the counting device 7 meansdetermination on displacement direction of the object 10. That is, ifthe counting result N′u when the oscillation wavelength of thesemiconductor laser 1 increases is equal to or larger than the countingresult N′d when the oscillation wavelength decreases (D(t)=1), thedisplacement direction of the object 10 is a direction in which theobject 10 approaches the semiconductor laser 1. If the counting resultN′u smaller than the counting result N′d (D(t)=0), the displacementdirection of the object 10 is the direction in which the object 10becomes further away from the semiconductor laser 1. Accordingly,essentially, if a cycle of the binarization output shown in FIG. 13C canbe obtained, the vibration frequency of the object 10 can be calculated.

The cycle measuring unit 82 measures a cycle of the binarization outputD(t) stored in the storage 80 (Step S3 in FIG. 5). FIG. 14 is a view forexplaining operation of the cycle measuring unit 82. In FIG. 14, H1 is athreshold value used to detect a rise in the binarization output D(t)and H2 is a threshold value used to detect a fall in the binarizationoutput D(t).

The cycle measuring unit 82 detects the rise in the binarization outputD(t) stored in the storage 80 by comparing the binarization output D(t)with the threshold value H1 and measures the cycle of the binarizationoutput D(t) by measuring a period tuu from the rise in the binarizationoutput D(t) to the next rise thereof. The cycle measuring unit 82performs such a measurement whenever a rising edge occurs in thebinarization output D(t).

Alternatively, the cycle measuring unit 82 may detect the fall in thebinarization output D(t) stored in the storage 80 by comparing thebinarization output D(t) with the threshold value H2 and measure thecycle of the binarization output D(t) by measuring a period tdd from thefall in the binarization output D(t) to the next fall thereof. The cyclemeasuring unit 82 performs such measurement whenever a falling edgeoccurs in the binarization output D(t).

A measurement result of the cycle measuring unit 82 is stored in thestorage 80. Next, the frequency distribution generator 83 generates afrequency distribution of cycles for a certain period of time T (T>Tt,for example, 100×Tt, i.e., a period corresponding to 100 triangularwaves) from the measurement result of the cycle measuring unit 82 (StepS4 in FIG. 5). FIG. 15 is a view showing one example of the frequencydistribution. The frequency distribution generated by the frequencydistribution generator 83 is stored in the storage 80. The frequencydistribution generator 83 generates such a frequency distribution everyperiod T.

Subsequently, the reference cycle calculator 84 calculates a referencecycle Tr, which is the representative value of the cycles of thebinarization output D(t), from the frequency distribution generated bythe frequency distribution generator 83 (Step S5 in FIG. 5). Arepresentative value of cycles is generally the mode or median, however,in this embodiment, the mode or median is inappropriate as therepresentative value of cycles. Therefore, the reference cyclecalculator 84 takes a class value giving the maximum product of theclass value and the frequency as the reference cycle Tr. The reason fortaking the class value giving the maximum product of the class value andthe frequency as the reference cycle Tr will be described later. A valueof the calculated reference cycle Tr is stored in the storage 80. Thereference cycle calculator 84 performs calculation of such a referencecycle Tr whenever the frequency distribution is generated by thefrequency distribution generator 83.

In the meantime, the counter 85 operates in parallel to the cyclemeasuring unit 82 and the frequency distribution generator 83 and countsthe number Na of rising edges of the binarization output D(t) (i.e., thenumber of pulses of “1” of the binarization output D(t)) for the sameperiod T as a period which is a target of the frequency distributiongeneration by the frequency distribution generator 83 (Step S6 in FIG.5). The counting result Na of the counter 85 is stored in the storage80. The counter 85 performs such a counting of the binarization outputD(t) every period T.

The corrector 86 obtains the total sum Nsa of frequencies of classes,which are equal to or smaller than ½ of the reference cycle Tr, and thetotal sum Nwa of frequencies of classes, which are equal to or largerthan 1.5 times the reference cycle Tr, from the frequency distributiongenerated by the frequency distribution generator 83, and corrects thecounting result Na of the counter 85 as follows (Step S7 in FIG. 5).

Na′=Na−Nsa+Nwa′  (7)

In Equation (7), Na′ is a corrected counting result. The correctedcounting result Na′ is stored in the storage 80. The corrector 86performs such correction every period T.

FIG. 16 is a schematic view showing the total sums Nsa and Nwa offrequencies. In FIG. 16, Ts is a class value of ½ of the reference cycleTr and Tw is class value of 1.5 times the reference cycle Tr. Of course,the classes shown in FIG. 16 are representative values of cycles. InFIG. 16, frequency distributions between the reference cycle Tr and Tsand between the reference cycle Tr and Tw are omitted for the purpose ofbrevity of description.

FIGS. 17A and 17B are views for explaining the principle of correctingthe counting result of the counter 85. FIG. 17A is a view showing thebinarization output D(t) and FIG. 17B is a view showing the countingresult of the counter 85 corresponding to FIG. 17A.

Although the cycle of the binarization output D(t) generally depends onthe vibration frequency of the object 10, pulses of the binarizationoutput D(t) appear with the same cycle. If the vibration frequency ofthe object 10 is invariable, however, an MHP waveform may be deficientor include waveforms, which must not be counted as signals, due tonoises. As a result, the waveform of the binarization output D(t) mayalso be deficient or include waveforms which must not be counted assignals, which may result in an erroneous counting result of pulses ofthe binarization output D(t).

If such a signal deficiency occurs, the cycle Tw of the binarizationoutput D(t) at a site where the deficiency occurs is approximately twotimes as large as the original cycle. That is, if the cycle of thebinarization output D(t) is equal to or larger than approximately twotimes the reference cycle Tr, then it may be determined that the signalis deficient. Therefore, assuming that the total sum Nwa of frequenciesof classes which are equal to or larger than the cycle Tw is the numberof deficiencies of the signal, such signal deficiency may be correctedby adding Nwa to the counting result Na of the counter 85.

In addition, the cycle Ts of the binarization output D(t) at a sitewhere an original signal is divided due to a spike noise or the like hastwo signals, i.e., a signal having a cycle smaller by ½ than theoriginal cycle and a signal having a cycle larger by ½ than the originalcycle. That is, if the cycle of the binarization output D(t) is equal toor smaller than about ½ of the reference cycle Tr, then it may bedetermined that the signal has been excessively counted. Therefore,assuming that the total sum Nsa of frequencies of classes which areequal to or smaller than the cycle Ts is the number of excessive signalcounts, noises erroneously counted may be corrected for by subtractingNsa from the counting result Na of the counter 85. Such are theprinciples of correcting the counting result shown in Equation (7).

The frequency calculator 87 calculates the vibration frequency fsig ofthe object 10 based on the corrected counting result Na′ calculated bythe corrector 86, as follows (Step S8 in FIG. 5).

Fsig=Na′/T  (8)

The display 9 displays a value of the vibration frequency fsigcalculated by the computing device 8.

Here, the principle of correcting the counting result of the counter 73of the counting device 7 will be described. The correction of thecounting result shown in Equation (2) has the same basic principle asthe correction of the counting result shown in Equation (7). However,according to the correction principle disclosed in JP-A-2009-47676, if aburst noise having a frequency higher than that of MHP is mixed into asignal input to the counting device, the counting result of the counter73 may not be properly corrected. In particular, the number of samplingsin a vibration frequency measurement may take only a periodcorresponding to several times or so the vibration frequency, andaccordingly, a small counting error may result in a large frequencyerror. Hereinafter, related-art problems will be described with anexample of a vibration frequency measurement.

FIGS. 18A to 18D are provided to explain problems of the related-artcounting device disclosed in JP-A-2009-47676. FIG. 18A is a view showingthe temporal change in the distance to the object 10, FIG. 18B is a viewshowing a temporal change of a speed of the object 10, FIG. 18C is aview showing a temporal change of the counting result of the countingdevice and FIG. 18D is a view showing the binarization output D(t)generated by binarizing the counting result of the counting device. InFIG. 18B, reference numeral 160 denotes a site having a low speed,reference numeral 161 denotes a direction in which the object 10approaches the semiconductor laser 1, and reference numeral 162 denotesa direction in which the object 10 gets far away from the semiconductorlaser 1. In addition, although FIGS. 18A to 18D shows a case where theratio of the maximum speed of the vibration of the object 10 to adistance to the object 10 is smaller than a change rate of wavelength ofthe semiconductor laser 1 in this embodiment, since the related-artcounting device has the same signal waveform as the inventive countingdevice, problems of the related-art counting device will be describedwith reference to FIGS. 18A to 18D.

As shown in FIG. 18C, if noise having a frequency higher than MHP isintroduced in a site 163 having a low speed of the object 10, amagnitude relationship between the counting results N′u and N′d may bethe reverse of the original relationship. As a result, as shown in FIG.18D, at a site 164 where the sign of the binarization output D(t) ischanged, the sign of the binarization output D(t) may have a valueinverse to the original value.

In a case where MHP is binarized with a threshold value, at a site whenMHP takes a value close to the threshold value, since the sign is likelyto be inverted due to noise having a high frequency and the site wherethe sign is likely to be inverted exists every ½ cycle of MHP, thefrequency distribution of MHP cycles includes a distribution 171 havingthe maximum value of frequencies with a cycle which is about half of theoriginal cycle Ta of MHP, and a cycle 172 having a short noise, inaddition to a distribution 170 having the maximum value of frequencieswith the original cycle Ta of MHP, as shown in FIG. 19. In addition, themaximum value of their frequencies tends to shift to a class having aslight short period due to mixed noise with a high frequency. Inaddition, in some cases noise with a high frequency may be continuouslymixed in. In the related-art counting device disclosed inJP-A-2009-47676, when such continuous high frequency noise is mixed, thecounting result of MHP cannot be sufficiently corrected.

In this embodiment, the counting result is corrected using not arepresentative value Ta of cycles of MHP but a representative value T0of half cycles. FIG. 20 shows an example of a frequency distribution ofhalf cycles of MHP. As can be seen from FIG. 20, when the frequencydistribution of half cycles of MHP is obtained, the maximum value offrequencies does not appear near 0.5 T0 even when high frequency noiseis mixed in a signal input to the counting device 7. That is, since themaximum value of frequencies near a threshold value to obtain the totalsum Ns of the number of half cycles which is less than ½ of therepresentative value T0 disappears, the Ns can be properly obtained tosuppress an error of correction. Such are the principles of correctingthe counting result shown in Equation (2). In addition, the reason forhalving the right side of Equation (2) is to convert the number of halfcycles of MHP into the number of MHPs.

As described above, in this embodiment, by counting the number of halfcycles of MHPs during the counting period by unit of the counter 73,measuring the half cycle of MHPs during the counting period, generatingthe frequency distribution of the half cycle of MHPs during the countingperiod from this measurement result, calculating the representativevalue T0 of the half cycles of MHPs from the frequency distribution,obtaining the total sum Ns of the number of half cycles which aresmaller than ½ of the representative value T0, and the total sum Nw_(n)of the number of half cycles, which are equal to or larger than 2n timesand smaller than (2n+2) times the representative value T0, andcorrecting the counting result of the counter 73 based on thefrequencies Ns and Nw_(n), since a counting error of MHPs can becorrected with high precision even when noise having a frequency higherthan that of MHP continuously occurs in a signal input to the countingdevice, it is possible to improve the measurement precision of thevibration frequency of the object 10.

In addition, in this embodiment, by binarizing the counting result ofMHPs by comparing magnitudes of the counting results of the first andsecond oscillation periods P1 and P2 temporally adjacent to each other,generating the frequency distribution of cycles for a certain period oftime T by measuring the cycles of the binarization output D(t),calculating the reference cycle Tf, which is the representative value ofthe distribution of the cycles of the binarization output D(t), from thefrequency distribution of cycles, counting the number of pulses of thebinarization output D(t) for the certain period of time T, obtaining thetotal sum Nsa of frequencies of classes which are equal to or smallerthan ½ of the reference cycle Tr and the total sum Nwa of frequencies ofclasses which are equal to or larger than 1.5 times the reference cycleTr from the frequency distribution, and correcting the counting resultof the pulses of the binarization output D(t) based on these frequenciesNsa and Nwa, since a counting error of the binarization output D(t) canbe corrected, it is possible to improve the measurement precision of thevibration frequency of the object 10.

Next, the reason why the reference cycle calculator 84 takes a classvalue giving the maximum product of the class value and the frequency asthe reference cycle Tr will be described.

For a self-coupling type laser measurement device using a wavelengthmodulation (triangular wave modulation in this embodiment), the numberof MHPs for each counting period is the sum of or the difference betweenthe number of MHPs which is proportional to the distance to the object10 and the number of MHPs which is proportional to a displacement(speed) of the object 10 for the counting period. According to amagnitude relationship between the ratio of the maximum speed ofvibration of the object 10 to the distance to the object 10 and a rateof change of a wavelength of the semiconductor laser 1, a signalobtained by the measurement device may be classified into two conditionsas follows.

First, a case where the ratio of the maximum speed of vibration of theobject 10 to the distance to the object 10 is smaller than the rate ofchange of the wavelength of the semiconductor laser 1 will be describedwith reference to FIGS. 18A to 18D. If the ratio of the maximum speed ofvibration of the object 10 to the distance to the object 10 is smallerthan the rate of change of the wavelength of the semiconductor laser 1,since the number of MHPs which is proportional to the distance to theobject 10 is always larger than the number of MHPs which is proportionalto the displacement (speed) of the object 10 for the counting period,the absolute value of the difference between the counting result N′uwhen the oscillation wavelength of the semiconductor laser 1 increasesand the counting result N′d when the oscillation wavelength decreases isalways proportional to the displacement of the object 10 for twocounting periods (the oscillation periods P1 and P2 in this embodiment).In this case, a plot of N′u−N′d in a time-series shows the speed ofvibration with the approach direction to the semiconductor laser 1 asthe positive direction. Accordingly, the sign of N′u−N′d represents themovement direction of the object 10 and the displacement of the object10 can be binarized using this sign.

At this time, the frequency distribution of cycles generated by thefrequency distribution generator 83 is as shown in FIG. 21.

If white noise, which may be caused by, for example, disturbance lightor the like, is applied to the site 163 having a low speed of the object10 as shown in FIG. 18C, the sign of the binarization output D(t) mayhave a value inverse to the original value at the site 164 where thesign of the binarization output D(t) is changed. In addition, if spikenoise, which may be caused by, for example, disturbance light or thelike, is applied, the sign of the binarization output D(t) is locallychanged at the site 165 as shown in FIG. 18D.

As a result, the frequency distribution of cycles generated by thefrequency distribution generator 83 becomes the sum of a normaldistribution 190 with the reference cycle Tr as the center, a frequency191 by sign inversion due to spike noise, and a frequency 192 by signinversion due to white noise, as shown in FIG. 21. In addition, afrequency 193 of sign deficiency when the binarization is performed doesnot frequently appear as long as low frequency noise with a high speedis not mixed.

Next, a case where the ratio of the maximum speed of vibration of theobject 10 to the distance to the object 10 is larger than the rate ofchange of the wavelength of the semiconductor laser 1 will be described.FIGS. 22A to 22D are views to explain signals obtained by the vibrationfrequency measurement device according to this embodiment, FIG. 22Abeing a view showing a temporal change in the distance to the object 10,FIG. 22B being a view showing a temporal change of a speed of the object10, FIG. 22C being a view showing a temporal change of the countingresult of the counting device 7 and FIG. 22D being a view showing thebinarization output D(t) generated by the binarizer 81. In FIG. 22B,reference numeral 220 denotes a site having a low speed, referencenumeral 221 denotes a direction in which the object 10 approaches thesemiconductor laser 1, and reference numeral 222 denotes a direction inwhich the object 10 gets far away from the semiconductor laser 1.

If the ratio of the maximum speed of vibration of the object 10 to thedistance to the object 10 is larger than the rate of change of thewavelength of the semiconductor laser 1, since the number of MHPs whichis proportional to the distance to the object 10 becomes smaller thanthe number of MHPs which is proportional to the displacement (speed) ofthe object 10 for the counting period near the maximum speed of theobject 10, there exist a period in which the difference between thecounting result N′u when the oscillation wavelength of the semiconductorlaser 1 increases and the counting result N′d when the oscillationwavelength decreases is proportional to the displacement of the object10 for two counting periods (the oscillation periods P1 and P2 in thisembodiment) and a period in which the sum of the counting result N′u andthe counting result N′d is proportional to the displacement of theobject 10 for the two counting periods.

In this case, the speed of vibration of the object 10 can be expressedby composition of graphs which plot N′u−N′d and N′u+N'd in atime-series, as shown in FIG. 22B. Here, since the direction of thespeed always matches the magnitude relationship between N′u and N′d, thesign of N′u−N′d represents the movement direction of the object 10 andthe displacement of the object 10 can be binarized using this sign.

Like the case where the ratio of the maximum speed of vibration of theobject 10 to the distance to the object 10 is smaller than the rate ofchange of the wavelength of the semiconductor laser 1, if white noise,which may be caused by, for example, disturbance light or the like, isapplied to a site 223 having a low speed of the object 10, the sign ofthe binarization output D(t) may have a value inverse to the originalvalue at a site 224 where the sign of the binarization output D(t) ischanged. In addition, if spike noise, which may be caused by, forexample, disturbance light or the like, is applied, the sign of thebinarization output D(t) is locally changed at a site 225 as shown inFIG. 22D. At this time, the frequency distribution of cycles generatedby the frequency distribution generator 83 is as shown in FIG. 21.

In correcting the binarization output D(t) generated by binarizing thedisplacement of the object 10 as in this embodiment, correction of highfrequency noise is important. A change of sign in a short period, whichmay be caused by high frequency noise, may exceed the frequency ofcycles of inherent vibration of the object 10, or, if the mode, themedian or the like is used as a representative value of cycles, acorrection may be applied by mistake on the basis of a noise cycleshorter than the vibration cycle. Accordingly, for a certain period oftime T for calculating a vibration frequency, a correction on thecounting result of the counter 85 is performed with a class value givingthe highest percentage of occupation of a signal having a class, i.e.,the maximum product of the class value and the frequency, as thereference cycle Tr. Such are the reason why a class value giving themaximum product of the class value and the frequency is taken as thereference cycle Tr.

This reason for taking a class value giving the maximum product of theclass value and the frequency as the reference cycle Tr is equallyapplied to the representative value calculator 742. That is, in a casewhere high frequency noise exists, it is more advantageous to take aclass value giving the highest percentage of occupation of a signalhaving a class as the representative value T0 for a counting period,rather than using the mode or the median as the representative value T0.

In addition, as another example of this embodiment, the technique forcorrecting the counting result of the counter 73 may be applied to atechnique for correcting the counting result of the counter 85.

Second Embodiment

Next, a second embodiment of the present invention will be described.FIG. 23 is a block diagram showing one example of a configuration of acounting device according to this embodiment. This embodiment uses acounting device 7 a instead of the counting device 7 of the firstembodiment. The counting device 7 a includes a binarizer 71, an AND gate72, a counter 73 a, a counting result corrector 74 a and a storage 75.

FIG. 24 is a block diagram showing one example of a configuration of thecounting result corrector 74 a of this embodiment. The counting resultcorrector 74 a includes a half cycle measuring unit 740, a frequencydistribution generator 741 a, a representative value calculator 742 a, acorrection value calculator 743 a and a signal combiner 744.

FIG. 25 is a flow chart showing operation of the counting device 7 aaccording to this embodiment. As described in the first embodiment, thehalf cycle measuring unit 740 measures half cycles of MHPs during acounting period (Step S101 in FIG. 25).

Like the first embodiment, the frequency distribution generator 741 agenerates a frequency distribution of the half cycles of MHPs during thecounting period from the measurement result of the half cycle measuringunit 740 stored in the storage 75 (Step S102 in FIG. 25).

Like the first embodiment, the representative value calculator 742 acalculates a representative value T0 of the MHP half cycle from thefrequency distribution generated in Step S102 by the frequencydistribution generator 741 a (Step S103 in FIG. 25). Like the firstembodiment, the mode, median or mean of the MHP half cycle may be takenas the representative value T0, or alternatively, a class value givingthe maximum of the product of the class value and the frequency may betaken as the representative value T0. The representative value T0calculated by the representative value calculator 742 a is stored in thestorage 75.

Next, for the measurement result of the half cycle measuring unit 740,the signal combiner 744 performs a process of taking a cycle, which is acombination of a half cycle smaller than ½ of the representative valueT0 and a half cycle measured immediately thereafter, as a half cycleafter combination and taking a waveform generated by combining cycles asa waveform corresponding to a half cycle of one MHP, until the halfcycle after combination reaches ½ or more of the representative value T0(Step S105 in FIG. 25). FIGS. 26A to 26C are provided to explainoperation of the signal combiner 744. FIG. 26A is a schematic viewshowing an MHP waveform, FIG. 26B is a view showing a measurement resultof the half cycle measuring unit 740 and FIG. 26C is a view showing theprocessing result of the signal combiner 744.

When the half cycle measuring unit 740 measures an MHP half cycle shownin FIG. 26A, measurement results, which are half cycles T1 to T16, areobtained as shown in FIG. 26B. Among them, the half cycles T2, T3, T6 toT9 and T11 to T14 are caused by high frequency noise or the like. Inthis case, since the half cycles T2, T3 and T6 to T14 are smaller than ½of the representative value T0, T10 is not recognized as the MHP halfcycle in the counting device 7 of the first embodiment, which causes anerror in the counting result.

On the contrary, in the second embodiment, when the signal combiner 744performs a combination process of the above signals, processing results,which are the half cycles T1 to T6, are obtained as shown in FIG. 26C.For example, a cycle which is a combination of the half cycles T2 to T4becomes the half cycle T2 after combination and waveforms of T2 to T4are combined into a waveform corresponding to one MHP half cycle.Similarly, a cycle which is a combination of the half cycles T6 to T10becomes the half cycle T4 after combination and waveforms of T6 to T10are combined into a waveform corresponding to one MHP half cycle. Theprocessing results of the signal combiner 744 are stored in the storage75.

Next, the frequency distribution generator 741 a generates a frequencydistribution of the half cycles of MHPs during the counting period fromthe processing results of the signal combiner 744 stored in the storage75 (Step S106 in FIG. 25).

Subsequently, the representative value calculator 742 a calculates therepresentative value T0 of the MHP half cycles from the frequencydistribution generated in Step S106 by the frequency distributiongenerator 741 a (Step S107 in FIG. 25). Accordingly, the representativevalue T0 stored in the storage 75 is updated as the newest valuecalculated in Step S107. Like the first embodiment, the mode, median ormean of the MHP half cycles may be taken as the representative value T0,or alternatively, a class value giving the maximum of the product of theclass value and the frequency may be taken as the representative valueT0.

In the meantime, the counter 73 a counts the number of half cycles ofthe MHPs processed by the signal combiner 744 (Step S108 in FIG. 25).

Finally, the correction value calculator 743 a obtains the total sum Nsof the number of half cycles, which are smaller than ½ of therepresentative value T0, and the total sum Nw_(n) of the number of halfcycles, which are equal to or larger than 2n times and smaller than(2n+2) times (n is a natural number equal to or larger than 1 and equalto or smaller than n_(max)) the representative value T0, from theprocessing results of the signal combiner 744 and corrects the countingresult N of the counter 73 a as expressed by the above Equation (2)(Step S109 in FIG. 25).

The counting device 7 a performs the above-described process for thefirst oscillation period P1 and the second oscillation period P2.

Other configurations are the same as the first embodiment. If loweringof the signal strength of MHP and a mixture of burst noise in a signalinput to the counting device 7 occur simultaneously, although a smallnumber of MHPs may be counted in the first embodiment, such countingerrors may be reduced according to the second embodiment.

In addition, although the frequency distribution of the half cycles ofMHPs during the counting period is generated in Step S102 and therepresentative value T0 of the half cycles of MHPs is calculated fromthe frequency distribution in Step S103 in the second embodiment, thepresent invention is not limited thereto but the representative valuecalculator 742 a may calculate the mean of half cycles of MHPs duringthe counting period as the representative value T0 from the measurementresult of the half cycle measuring unit 740 in Step S103 withoutgenerating the frequency distribution in Step S102.

Third Embodiment

Next, a third embodiment of the present invention will be described. Thethird embodiment has the same configuration and process flow of thecounting device as the second embodiment, and therefore will bedescribed using reference numerals of FIGS. 23 to 25.

Steps S101 to S103 of FIG. 25 have the same process as those of thesecond embodiment.

Next, for the measurement result of the half cycle measuring unit 740,if a half cycle smaller than ½ of the representative value T0 liesbetween an m-th half cycle Tm which is equal to or larger than ½ of therepresentative value T0 and a p-th half cycle Tp which is equal to orlarger than ½ of the representative value T0 (m and p are naturalnumbers), the signal combiner 744 of this embodiment takes a cycle,which is a combination of from the half cycle Tm to the half cycle Tp,as a half cycle after combination if (m+p) is an even number, takes acycle, which is a combination of from the half cycle Tm to a half cycleTn−1, as a half cycle after combination if (m+p) is an odd number, andtakes a waveform generated by combining cycles as a waveformcorresponding to the m-th half cycle (Step S105 in FIG. 25).

FIGS. 27A to 27C are provided to explain operation of the signalcombiner 744 of this embodiment, FIG. 27A being a schematic view showingan MHP waveform, FIG. 27B being a view showing a measurement result ofthe half cycle measuring unit 740 and FIG. 27C being a view showing aprocessing result of the signal combiner 744.

When the half cycle measuring unit 740 measures an MHP half cycle shownin FIG. 27A, measurement results, which are half cycles T1 to T20, areobtained as shown in FIG. 27B. In this case, since the half cycles T2,T3, T6 to T14 and T16 to T19 are smaller than ½ of the representativevalue T0, T10 is not recognized as the MHP half cycle in the countingdevice 7 of the first embodiment, which causes an error in the countingresult.

On the contrary, in the third embodiment, when the signal combiner 744performs a combination process of the above signals, processing results,which are the half cycles T1, T2, T3 and T4, are obtained as shown inFIG. 27C. For example, the half cycles T2 and T3 lie between the halfcycle T1 and the half cycle T4 which are equal to or lager than ½ of therepresentative value T0 and (m+p) is an odd number (5=1+4). Accordingly,waveforms of T1 to T3 are combined into a waveform corresponding to oneMHP half cycle and a cycle, which is a combination of the half cycles T1to T3, becomes the half cycle T1 after combination.

Similarly, the half cycles T6 to T14 lie between the half cycle T5 andthe half cycle T15 which are equal to or lager than ½ of therepresentative value T0 and (m+p) is an even number (20=5+15).Accordingly, waveforms of T5 to T15 are combined into a waveformcorresponding to one MHP half cycle and a cycle, which is a combinationof the half cycles T5 to T15, becomes the half cycle T3 aftercombination. In addition, the half cycles T16 to T19 lie between thehalf cycle T3 after combination and the half cycle T20 which is equal toor lager than ½ of the representative value T0 and (m+p) is an oddnumber (23=3+20). Accordingly, waveforms of the half cycles T3 and T16to T19 are combined into a waveform corresponding to one MHP half cycleand a cycle, which is a combination of the half cycles T3 and T16 toT19, becomes the half cycle T3 after combination. The processing resultsof the signal combiner 744 are stored in the storage 75.

Steps S106 to S109 of FIG. 25 have the same process as those of thesecond embodiment. Although a counting error may be lessened as comparedto the first embodiment, if burst noise or popcorn noise having a cyclewhich is equal to or larger than a ¼ cycle of MHP is mixed in a signalinput to the counting device 7 a, the burst noise or the popcorn noisemay be counted in the second embodiment, which may cause a countingerror. On the contrary, such counting errors may be reduced according tothe third embodiment even if such a mixture of noise occurs.

In addition, like the second embodiment, the representative valuecalculator 742 a may calculate the mean of half cycles of MHPs duringthe counting period as the representative value T0 from the measurementresult of the half cycle measuring unit 740 in Step S103 withoutgenerating the frequency distribution in Step S102.

In addition, the processes of Steps S106 and S107 are not requisite forthe second and third embodiments. The reason for this is that there isno need to obtain a representative value again since the representativevalue before combination can be obtained with high precision using thefrequency distribution. If the processes of Steps S106 and S107 are notperformed, the correction value calculator 743 a may use therepresentative value T0 calculated in Step S103. However, if it isconsidered that the representative value T0 calculated in Step S103 haslow precision, the processes of Steps S106 and S107 may be performed.

Fourth Embodiment

Next, a fourth embodiment of the present invention will be described.FIG. 28 is a view showing one example of a configuration of a countingdevice according to this embodiment. This embodiment uses a countingdevice 7 b instead of the counting device 7 of the first embodiment. Thecounting device 7 b includes a binarizer 71, an AND gate 72, a counter73 b, a counting result corrector 74 b and a storage 75.

FIG. 29 is a block diagram showing one example of a configuration of thecounting result corrector 74 b of this embodiment. The counting resultcorrector 74 b includes a cycle measuring unit 745, a frequencydistribution generator 741 b, a representative value calculator 742 b, acorrection value calculator 743 b and a signal combiner 744 b.

FIG. 30 is a flow chart showing operation of the counting device 7 baccording to this embodiment. The cycle measuring unit 745 measurescycles of MHPs during a counting period (Step S201 in FIG. 30).Specifically, the cycle measuring unit 745 detects a rise in the outputof the AND gate 72 while detecting a fall in the output of the AND gate72. In addition, the cycle measuring unit 745 measures a cycle of theoutput (i.e., MHP cycle) of the AND gate 72 during the counting periodby measuring a period from the rise in the output of the AND gate 72 tothe next rise thereof. The cycle measuring unit 745 performs suchmeasurement whenever a rising edge occurs in the output of the AND gate72. Alternatively, the cycle measuring unit 745 may measure the MHPcycle by measuring a period from the fall in the output of the AND gate72 to the next fall thereof. The storage 75 stores the measurementresult of the cycle measuring unit 745.

After a gate signal GS falls and the counting period is ended, thefrequency distribution generator 741 b generates a frequencydistribution of the cycles of MHPs during the counting period from themeasurement result of the cycle measuring unit 745 stored in the storage75 (Step S202 in FIG. 30).

Subsequently, the representative value calculator 742 b calculates arepresentative value T0 of the MHP cycles from the frequencydistribution generated by the frequency distribution generator 741 b(Step S203 in FIG. 30). Here, the mode, median or mean of the MHP cyclesmay be taken as the representative value T0. Alternatively, therepresentative value calculator 742 b may take a class value giving themaximum of the product of the class value and the frequency as therepresentative value T0.

Next, for the measurement result of the cycle measuring unit 745, thesignal combiner 744 b performs a process of taking a cycle, which is acombination of a cycle smaller than ½ of the representative value T0 anda cycle measured immediately thereafter, as a cycle after combinationand taking a waveform generated by combining cycles as a waveformcorresponding to one cycle of one MHP, until the cycle after combinationreaches ½ or more of the representative value T0 (Step S204 in FIG. 30).FIGS. 31A to 31C are views to explain operation of the signal combiner744 b. FIG. 31A is a schematic view showing an MHP waveform, FIG. 31B isa view showing a measurement result of the cycle measuring unit 745 andFIG. 31C is a view showing a processing result of the signal combiner744 b.

When the cycle measuring unit 745 measures an MHP cycle shown in FIG.31A, measurement results, which are cycles T1 to T7, are obtained asshown in FIG. 31B. Among them, the cycles T1, T3, T4, T6 and T7 arecaused by high frequency noise or the like. In this case, since thecycles T1 and T3 to T6 are smaller than ½ of the representative valueT0, an error of the counting result occurs at sites T3 to T7 in thecounting device 7 of the first embodiment.

On the contrary, in the fourth embodiment, when the signal combiner 744b performs a combination process of the above signals, processingresults, which are the cycles T1 and T2, are obtained as shown in FIG.31C. For example, a cycle which is a combination of the cycles T1 and T2becomes the cycle T1 after combination and waveforms of T1 and T2 arecombined into a waveform corresponding to one cycle of one MHP. Here,the waveforms of T1 and T2 are combined such that the cycle aftercombination is equal to or larger than ½ of the representative value T0.Similarly, a cycle which is a combination of the cycles T3 to T7 of FIG.31B becomes the cycle T2 after combination as shown in FIG. 31C andwaveforms of T3 to T7 are combined into a waveform corresponding to onecycle of one MHP. The processing results of the signal combiner 744 bare stored in the storage 75.

Next, the frequency distribution generator 741 b generates a frequencydistribution of the cycles of MHPs during the counting period from theprocessing results of the signal combiner 744 b stored in the storage 75(Step S205 in FIG. 30).

Subsequently, the representative value calculator 742 b calculates therepresentative value T0 of the MHP cycles from the frequencydistribution generated in Step S205 by the frequency distributiongenerator 741 b (Step S206 in FIG. 30). Accordingly, the representativevalue T0 stored in the storage 75 is updated as the newest valuecalculated in Step S206. Like Step S203, the mode, median or mean of theMHP cycles may be taken as the representative value T0, oralternatively, a class value giving the maximum of the product of theclass value and the frequency may be taken as the representative valueT0.

In the meantime, the counter 73 b counts the number of MHPs processed bythe signal combiner 744 b (Step S207 in FIG. 30). Although the counter73 of the first embodiment counts both of the rise and fall of MHPs, thecounter 73 b has only to count one of the rise and fall of MHPs.

Finally, the correction value calculator 743 b obtains the total sum Nsof the number of cycles, which are smaller than ½ of the representativevalue T0, and the total sum Nw_(n) of the number of cycles, which areequal to or larger than (n+0.5) times and smaller than (n+1.5) times (nis a natural number equal to or larger than 1 and equal to or smallerthan n_(max)) the representative value T0, from the processing resultsof the signal combiner 744 b and corrects the counting result of thecounter 73 b as expressed by the following Equation (Step S208 in FIG.30).

$\begin{matrix}{{N^{\prime} = {N - {Ns} + {\sum\limits_{n = 1}^{n_{\max}}\left( {n \times {Nw}_{n}} \right)}}}{n_{\max} \leqq \frac{T_{\max}}{T\; 0}}} & (9)\end{matrix}$

In Equation (9), N is the number of MHPs, which is the counting resultof the counter 73 b, N′ is the counting result after correction, andT_(max) is the maximum value which can be taken by the MHP cycle.

The counting device 7 b performs the above-described process for thefirst oscillation period P1 and the second oscillation period P2.

Other configurations are the same as the first embodiment. The fourthembodiment can not obtain an effect of correcting the counting resultusing a representative value of half cycles as in the first embodiment,but, as described in the second embodiment, can reduce counting errorseven if lowering of the signal strength of MHP and a mixture of burstnoise in a signal input to the counting device 7 b occur simultaneously.

In addition, although the frequency distribution of the cycles of MHPsduring the counting period is generated in Step S202 and therepresentative value T0 of the cycles of MHPs is calculated from thefrequency distribution in Step S203 in the fourth embodiment, thepresent invention is not limited thereto but the representative valuecalculator 742 b may calculate the mean of cycles of MHPs during thecounting period as the representative value T0 from the measurementresult of the cycle measuring unit 745 in Step S203 without generatingthe frequency distribution in Step S202.

In addition, the processes of Steps S205 and S206 are not requisite forthe fourth embodiment. The reason for this is that there is no need toobtain a representative value again since the representative valuebefore combination can be obtained with high precision using thefrequency distribution. If the processes of Steps S205 and S206 are notperformed, the correction value calculator 743 b may use therepresentative value T0 calculated in Step S203. However, if it isconsidered that the representative value T0 calculated in Step S203 haslow precision, the processes of Steps S205 and S206 may be performed.

In addition, at least the counting devices 7, 7 a and 7 b and thecomputing device 8 in the first to fourth embodiments may be implementedby a computer including, for example, a CPU, a storage and an interfaceand a program for controlling these hardware resources. The program foroperating such a computer is provided in a state where the program isrecorded on a recording medium such as a flexible disk, a CD-ROM, aDVD-ROM, a memory card or the like. The CPU writes a read program in thestorage and executes the processes described in the first to fourthembodiments according to the program.

In addition, although it has been illustrated in the first to fourthembodiments that the counting devices of the present invention areapplied to a vibration frequency measurement device, the presentinvention is not limited thereto but the counting devices of the presentinvention may be applied to other various fields. The counting devicesof the present invention can be effectively utilized when the number ofsignals to be counted has a linear relationship with particular physicalquantities (the distance between the semiconductor laser 1 and theobject 10 and the displacement of the object 10 in the first to fourthembodiments) and the signals have substantially the single frequency ifthe particular physical quantities are constant.

In addition, even if the signals have no single frequency, the countingdevices of the present invention are effectively utilized withsubstantially a single frequency even when an enlargement of a cycledistribution is small for a particular physical quantity, such as aspeed of an object which is vibrating at a frequency sufficiently lowerthan the counting period, for example, a frequency which is equal to orlower than 1/10 of the counting period.

In addition, a physical quantity sensor has been illustrated with thevibration frequency measurement device in the first to fourthembodiments, but, without being limited thereto, the present inventionmay be applied to other various physical quantity sensors. For example,the tension of an object may be calculated from the counting result ofthe counting devices, or the distance to an object and the speed of theobject may be calculated from the counting result of the countingdevices as disclosed in Patent Document 1. As can be seen from the factthat the physical quantity sensor can calculate various physicalquantities, the above-mentioned particular physical quantities may beequal to or different from the physical quantities calculated by thephysical quantity sensor.

The present invention is applicable to a counting device which countsthe number of signals and an interference type physical quantity sensorwhich obtains physical quantities of an object to be measured bymeasuring the number of interference waveforms using the countingdevice.

While the present invention has been shown and described with referenceto certain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. It is aimed, therefore, to cover in theappended claim all such changes and modifications as fall within thetrue spirit and scope of the present invention.

1-6. (canceled)
 7. A counting device which counts the number of signalshaving a linear relationship with a specific physical quantity, whereinthe signals have substantially a single frequency if the specificphysical quantity is constant, the device comprising: a signal cyclemeasurement unit configured to measure cycles of input signals duringgiven counting periods whenever a signal is input; a frequencydistribution generator configured to generate a frequency distributionof the cycles of the input signals during the given counting periods,based on a measurement result from the signal cycle measurement unit; arepresentative value calculator configured to calculate a representativevalue of a distribution of the cycles of the input signals, based on thefrequency distribution; a signal combination unit configured to combinea first cycle that is less than 0.5 times the representative value witha cycle measured immediately after the first cycle so as to set thecombined cycle as a new cycle of a signal waveform, wherein when thecombined cycle is less than 0.5 times the representative value, thesignal combination unit continues to perform the combination processuntil newly-combined cycle is equal to or greater than 0.5 times therepresentative value; a signal counter configured to count the number ofthe cycles of the input signals during the given counting period,wherein the number of the cycles contains the new half cycles combinedby the signal combination unit; and a correction value calculatorconfigured to calculate, based on the measurement result from the signalcycle measurement unit, a total number Ns and a total number Nw_(n) soas to correct the number of the cycles of the input signals counted bythe signal counter, based on the total number Ns and the total numberNw_(n), wherein Ns represents the total of the number of the half cyclesthat are less than 0.5 times the represent value, and Nw_(n) representsthe total of the number of the half cycles that are equal to or greaterthan (n+0.5) and less than (n+1.5) times the representative value, wheren is a natural number of 1 or more.
 8. The device according to claim 7,wherein the correction value calculator is configured to calculate thecorrected number N′ of the cycles of the input signals, based on thefollowing expression:$N^{\prime} = {N - {Ns} + {\sum\limits_{n = 1}^{n_{\max}}\left( {n \times {Nw}_{n}} \right)}}$$n_{\max} \leqq \frac{T_{\max}}{T\; 0}$ wherein the number of thecycles of the input signals counted by the signal counter is N, therepresentative value is T0 and the maximum value of the cycles of theinput signals is T_(max).
 9. The counting device according to claim 7,wherein the representative value is one of the median, the mode, themean and a class value, the class value having a maximum value obtainedby multiplying the class value by a frequency corresponding to the classvalue. 10-16. (canceled)
 17. A method of counting the number of signalshaving a linear relationship with a specific physical quantity, whereinthe signals have substantially a single frequency if the specificphysical quantity is constant, the method comprising: (a) measuringcycles of input signals during given counting periods whenever a signalis input; (b) generating a frequency distribution of the cycles of theinput signals during the given counting periods, based on a measurementresult in step (a); (c) calculating a representative value of adistribution of the cycles of the input signals, based on the frequencydistribution; (d) combining a first cycle that is less than 0.5 timesthe representative value with a cycle measured immediately after thefirst cycle so as to set the combined cycle as a new cycle of a signalwaveform, wherein when the combined cycle is less than 0.5 times therepresentative value, wherein step (d) comprises: continuing to performthe combination process until newly-combined cycle is equal to orgreater than 0.5 times the representative value; (e) counting the numberof the cycles of the input signals during the given counting period,wherein the number of the cycles contains the new half cycles combinedin step (d); and (f) calculating, based on the measurement result instep (a), a total number Ns and a total number Nw_(n) so as to correctthe number of the cycles of the input signals counted in step (e),wherein Ns represents the total of the number of the half cycles thatare less than 0.5 times the represent value, and Nw_(n) represents thetotal of the number of the half cycles that are equal to or greater than(n+0.5) and less than (n+1.5) times the representative value, where n isa natural number of 1 or more.
 18. The counting method according toclaim 17, wherein step (f) comprises: calculating the corrected numberN′ of the half cycles of the input signal, based on the followingexpression:$N^{\prime} = {N - {Ns} + {\sum\limits_{n = 1}^{n_{\max}}\left( {n \times {Nw}_{n}} \right)}}$$n_{\max} \leqq \frac{T_{\max}}{T\; 0}$ wherein the number of thecycles of the input signals counted in step (e) is N, the representativevalue is TO and the maximum value of the cycles of the input signals isT_(max).
 19. The counting method according to claim 17, wherein therepresentative value is one of the median, the mode, the mean and aclass value, the class value having a maximum value obtained bymultiplying the class value by a frequency corresponding to the classvalue.
 20. (canceled)